How can I make Acrobat save the scale (e.g. 1:50) when I save a document?

Hello,

I have a document (a house plan) whose scale is 1:50. I regularly need to load up this document and take measurements off it, using the Measuring Tool. But each time I load it up, the scale is reset to 1:1, so I have to right-click and reset it to 1:50. Even if I then save the document, it forgets the scale and goes back to 1:1 the next time I open it. Is there a way to fix this, or is it a bug in Acrobat?

Thanks in advance for any suggestsion!

Paul